Well, if you measured it and the result was between 42 and 43mm -
which translates to 1.6535 and 1.6929 and the exact middle of that being 1.6732 -
I think you may easily leave it to chance whether you'll find a 1.66 or a 1.67 filter first.
I cannot really imagine that in the "Elgeet times" there could have been reliable filter
thread measures other than these (that is with two digits behind the 'comma').
Cheers, Phil